1. Field of the Invention
This invention relates to a transmission method and a transmission apparatus suitable for application to wireless transmission processing in which retransmission control is performed.
2. Description of the Related Art
Channel encoding (error-correction encoding) and automatic repeat request (ARQ) are known as methods of error control in wireless transmission applied to wireless telephone communication and other systems. In packet transmission, error-free transmission is necessary, so that error control must be performed using ARQ. Further, when applying adaptive modulation/demodulation and error correction to select the optimum modulation method and encoding method according to the propagation path conditions in order to improve throughput, packet errors arising due to measurement errors, control delays and other causes are unavoidable; hence as described in non-patent reference 1, a technology called HARQ (Hybrid ARQ) which incorporates error correction functions has been developed.
HARQ is a retransmission method in which retransmission using ARQs provided by an upper layer of a conventional wireless protocol or similar is executed by the physical layer, and is further combined with error correction functions to be performed. By this means, the reliability of data provided to the upper layer by the physical layer can be improved. In ARQ retransmission technology in the upper layer, conventionally data for which there has been reception failure is discarded, and data retransmission is awaited; however in HARQ, data for which there has been reception failure is held in an error corrector as data before decoding, and is synthesized with retransmitted data to perform decoding. By synthesizing data for which reception has failed and retransmitted data, the data likelihood, which indicates the reliability of the data, can be improved, and the probability of successful decoding can be increased.
Here, an example of wireless data communication to which HARQ is applied is explained. As wireless data communication to which HARQ is applied, it is proposed that high-speed downlink packet access (hereafter HSDPA), which enables high-speed downlink data transmission to be performed, be applied for example to a universal mobile telephone communication system (UMTS system) to which W-CDMA has been applied.
In HSPDA, protocol data units (hereafter “PDUs”) which have been correctly decoded using HARQ functions are rearranged in order of the transmission sequence number (hereafter “TSN”) included in the PDU header information, and are transferred to the upper-layer protocol. HARQ processing and TSN rearrangement functions before transfer to the upper layer are obtained in a MAC (Media Access Control) layer. Protocol data units (PDUs) in the MAC layer are called MAC-PDUs. MAC-PDUs may be MAC-hs PDUs for the portion relating to the HSDPA method, and MAC-d PDUs for individual data transmission channels (DCHs) existing in the UMTS system before the introduction of HSDPA; and MAC-hs PDUs are configured to include a plurality of MAC-d PDUs.
An example of the layer configuration in this HSDPA method is explained, referring to FIG. 1. In FIG. 1, solid lines indicate data flows, and broken lines indicate control information flows. As shown in FIG. 1, there is a radio link control (hereafter “TRLC”) layer 1 in the layer above the MAC layer, which is the layer relating to HARQ functions; as the MAC layer, a MAC-d layer 2, MAC-c/sh layer 3, and MAC-hs layer 4 are prepared. Below the MAC layers, a transport channel layer 5 is provided.
The transport channel layer 5 has a function for receiving data from the physical layer, and a function for transmitting data to the physical layer. The MAC-hs layer 4 transfers MAC-hs PDUs in HSDPA to the MAC-d layer 2 in order. In the MAC-c/sh layer 3, data received through a common/shared channel is transferred to the MAC-d layer 2. The MAC-d layer 2 transfers MAC-d PDUs to the RLC layer 1. Each of the MAC layers also exchanges control information with the RLC layer 1, as indicated by broken lines in FIG. 1. The MAC-c/sh layer is not directly related to the processing of the present invention described later on, and therefore the specific processing performed is not here explained.
FIGS. 2A to 2E show the relations between protocol data units (PDUs) in HSPDA, that is, the relations between MAC-hs PDUs, MAC-d PDUs, and RLC PDUs. Data supplied as shown in FIG. 2A is divided into data segments in predetermined data quantity units as shown in FIG. 2B, and an RLC (Radio Link Control) header is added to each data segment. SN (Sequence Number) values for rearranging RLC PDUs (RLC Protocol Data Units) in order are included in the RLC header portions. The data to which the RLC header is added is sent to the MAC-d layer as an RLC PDU, and the MAC header is added, as shown in FIG. 2C.
The data to which the MAC header has been added is sent, as a MAC-d PDU (MAC-d protocol data unit), to the MAC-hs layer; as shown in FIG. 2D, a MAC-hs header is added to each predetermined unit, and as shown in FIG. 2E, the data is sent to the lower layer (transport channel layer) as a MAC-hs PDU (MAC-hs protocol data unit), and transmission processing is performed. At the time of reception, processing is performed in the direction opposite to the flow from FIGS. 2A to 2E, data units of the different protocols are identified in the data obtained from the transport channel layer, and the data shown in FIG. 2A is extracted.
Non-patent reference 1: Keiji Tachikawa, W-CDMA Idou Tsuushin Houshiki, Maruzen Shuppan, Jun. 25, 2001, pp. 48-50.
However, in an actual mobile wireless communication system, data transmission errors normally occur due to changes in and degradation of the radio wave propagation environment, as a result of fading, multipath interference, and other causes. As a result, at the receiving-side terminal, PDUs in which TSN (Transmission Sequence Number) and SN are missing occur in the reception portions of the MAC-hs layer and RLC layer, and so buffering is performed until the retransmitted PDUs can be received.
In the HSPDA method, it is stipulated that this buffering be managed in the MAC-hs layer through TSN values and a timer. In the MAC-hs layer, if MAC-hs PDUs have been received within the stipulated TSN window, and TSN values have been received in order, then these are passed to the MAC-d layer. When a MAC-hs PDU is received with a missing TSN value in the TSN window, the timer T1 is started with respect to the MAC-hs PDU with the data unit for the sequence number in the missing state.
The timer T1 is a dedicated timer managed by the MAC-hs layer, in which the timer value is set by the UMTS system to values from, for example, 10 ms to 400 ms, and issues instructions to terminals. The timer T1 has settings, in which reception of a MAC-hs PDU with missing TSN value is awaited until the end of the stipulated active time of the timer T1; however, when the timer T1 finishes counting the stipulated time, the MAC-hs layer abandons reception of MAC-hs PDUs with missing TSN value, and the MAC-hs PDUs remaining in the buffer are passed to the MAC-d layer.
On the other hand, in the RLC (Radio Link Control) layer, it is stipulated that management be performed using the SN (Sequence Number) values in RLC headers, retransmit timer, and retransmit times upper-limit count value. In the RLC layer, if RLC PDUs can be received within the stipulated SN window and the reception is performed in the order of SN values, then the data is passed to the upper-layer application. The SN window is set by the UMTS system to a value of for example 1 to 4095, issuing instructions to terminals. In the RLC layer of a receiving terminal, when an RLC PDU with a missing SN value is detected, a retransmission request is sent to the transmitting side using the control PDU. In the RLC layer on the transmitting side, if the retransmission request is accepted, the PDU, specified using the missing SN number, is retransmitted. It is further stipulated that, if the retransmission request at the receiving-side terminal or the retransmission on the transmitting side fails after a stipulated number of executions, then the received data accumulated in the buffer is discarded in the RLC layer. As a result, in cases where the radio wave environment is degraded, there is the problem that a vicious circle may occur in buffer management, with a state arising in which data accumulation and data discarding are repeated in the buffer of the RLC layer.
An example of the state in which the above problem occurs is explained, referring to FIGS. 3A to 3J. FIGS. 3A through 3E describe the states of the terminal on the data receiving side; FIGS. 3F through 3J describe the states of the base station which is the data transmitting side.
In FIGS. 3A to 3J, the horizontal axis is the time axis, and the transmission time intervals (hereafter called “TTIs”) of the physical channels are stipulated by the UMTS system. For example, values of 2 ms for HSDPA-related physical channels, and of 10 ms, 20 ms, 40 ms, or 80 ms are stipulated for individual channels (hereafter “DCHs”). In the example of FIGS. 3A to 3J, the TTI for the uplink DCH, for transmission to a base station, is set to 10 ms. Time differences are set as offsets determined by the system for the TTI boundaries of the physical channels. The SN (Sequence Number) window in the RLC layer may be for example 2047, a value actually used in services, and is set to a considerably large number.
As shown in FIGS. 3G and 3H, in the base station-side RLC layer, transmission RLC PDUs within the SN window are sent in succession to the transmission MAC-hs layer; the PDUs are processed as MAC-hs PDUs to be arranged at predetermined positions of the transmission channel at 2 ms intervals as shown in FIG. 3I; and are transmitted. The positions (intervals) of arrangement in the channel are determined depending on each transmission destination terminal.
Signals transmitted from the base station are received at predetermined positions of the receiving channel at the 2 ms intervals indicated in FIG. 3C. When it is assumed that at this time data have not been received (decoded) correctly on the receiving side, as indicated by an “x” symbol in FIGS. 3A to 3J. If data have not been received correctly, reception-not-acknowledged (NACK) is transmitted to the base station from the terminal on the transmitting channel, as shown in FIG. 3D. When transmission of data on the receiving channel is begun, the timer T1 is started.
NACK data transmitted from the terminal is received by the base station on the physical receiving channel, as shown in FIG. 3F. When this NACK data is received by the base station, the MAC-hs PDU with the same sequence number (SN) is resent with the timing allocated to the terminal which is the transmission target, as shown in FIG. 3I.
At this time, as indicated by an “x” symbol in FIGS. 3A to 3J, if correct data reception (decoding) has not been made either on the reception side with respect to the resent retransmitted MAC-hs PDU, similarly to the previous case, NACK data is transmitted from the terminal to the base station; here however, as indicated by the “x” symbol in the figures, it is assumed that the uplink transmission of the NACK data results in an error, so that the NACK data cannot be discriminated at the base station. In this case, the base station does not retransmit the MAC-hs PDU.
Upon entering such a state, as indicated in FIG. 3C, the data with the sequence number (SN) in question cannot be received before the end of counting by the timer T1 which has been started, and as indicated in FIG. 3B, the MAC-hs PDUs accumulated in the receiving buffer are sent to the RLC layer. In the RLC layer, as shown in FIG. 3A, because there is an omission among the transferred PDUs, a control PDU including a retransmission request is transmitted on the transmission channel from the terminal to the base station, as indicated in FIG. 3E.
On the base station side, as shown in FIG. 3J, upon receiving the control PDU including the retransmission request, processing to retransmit the RLC PDU is performed in the RLC layer as shown in FIG. 3G; a MAC-hs PDU is generated in the MAC-hs layer as shown in FIG. 3H, and as shown in FIG. 3I, this MAC-hs PDU is transmitted on the transmission channel.
Thereafter, if this transmitted PDU likewise cannot be normally received (decoded) on the terminal side, retransmission processing is repeated in the RLC layer of the base station, and data accumulated in the buffer is discarded in the RLC layer on the terminal side.
Note that in the example of FIGS. 3A to 3J, in order to make the explanation simplified, a case is shown in which processing is repeated upon failure to retransmit one MAC-hs PDU, indicated by the diagonal shading in FIG. 3I; however in practical use, retransmission processing is performed in parallel for a plurality of MAC-hs PDUs simultaneously.
Thus when MAC-hs PDU retransmission is repeated and uplink NACK transmission ends in failure, the timer T1 is started again and again, and when the timer which is frequently started completes counting, the received MAC-hs PDUs which are buffered in the MAC-hs layer of the terminal are passed to the MAC-d layer and RLC layer, with TSNs (Transmission Sequence Numbers) still discontinuous.
As already explained, the buffer (SN window) in the RLC layer is set to a comparatively large volume; for example, in a UMTS system, the SN window is set so as to handle 2047 PDUs. If sequence numbers (SN) are missed in succession in this SN window, the number of PDUs for RLC retransmission requests similarly increases significantly. When there is degradation of the radio wave environment, even if RLC retransmission is performed, there is a high probability that missing SNs will not be resolved within the RLC stipulated time. Further, if one missing sequence number cannot be resolved, RLC data for subsequent sequence numbers which have been received correctly is discarded, so that data transmission efficiency is significantly degraded. That is, in cases in which the radio environment is degraded, a vicious circle may be entered in which data accumulation in the RLC buffer and data discarding are repeated. As a result, an HSDPA service characterized by high-speed data transmission can no longer be obtained.
This invention was devised in light of the above problems, and has as an object to improve data transmission efficiency under circumstances of a poor radio wave environment in this type of wireless transmission system.